Bare spots. Yellow spots. Thin, scraggly growth. Your lawn needs help, but you don't know where or how to begin. Most lawn diseases are the result, say Cornell Extension horticulturists, of naturally occurring pathogens, usually fungi, that are present in your lawn at all times. When environmental conditions cooperate, fungi will proliferate, spread and damage your lawn. To combat lawn disease, experts suggest that you water in the early morning, avoid over-fertilizing, fill in water-collecting low spots and plant disease-resistant grass strains.
Diseases of Cold, Wet Conditions
Red thread favors cool, humid weather and causes bleached-out, pinkish cast to large areas of fescue or ryegrass lawns. In early spring, melting snow may reveal two types of snow molds: typhula blight and fusarium patch. Typhula blight causes straw-colored, circular patches of damaged grass, while fusarium patch appears off-white or pink.
Diseases of Hot, Humid Conditions
Leaf spot is a common lawn disease characterized by purple or tan spots on leaf blades. Leaf tips may die. Affected patches may be several inches to several feet across. Pythium causes irregularly shaped, 1- to 6-inch diameter spots. Grass turns light tan, dies and becomes shriveled. Necrotic ring spot affects Kentucky bluegrass, causing patches 6 to 24 inches in diameter. Roots and crowns will be covered with strands of black fungus. Summer patch is characterized by 6- to 12-inch spots of bronzed, dead grass. These spots may fill in with weedy growth. Cobwebby, tan-colored areas of turf may be the result of dollar spot, while rust causes dusty, orange-red patches to appear on leaf blades.
Other Causes of Damage
Dog-urine burn can mimic many of the patch-type lawn diseases. Small areas of dead grass surrounded by deep green growth typify urine spots. Flush areas with water after your dog urinates to control these patches.
Dull mower blades can shred rather than cut blades, causing the tips to brown. Cutting the grass too short can lead to "scalping," exposing the growth crown and killing the plant.